The invention relates to treads intended in particular to be fitted on tires and more particularly to the tread patterns of these treads. It also relates to moulds for manufacturing these treads.
In order to improve the grip performance during travel of a tire provided with a tread, it is known to provide this tread with motifs in relief, said motifs themselves being provided with incisions in order to increase the number and length of active ridges in the contact area between this tire and the ground. “Incision” is understood to mean grooves of low width relative to the width of the tread (for example, for a heavy-vehicle tire, an incision having a width less than 1.5 mm). When these incisions have depths substantially equal to the thickness of the motifs in relief, it was noted that the more the number of incisions in contact increases, the more the emission of noise during travel increases.
In order to reduce the level of noise emitted during travel of a tire, the tread of which comprises incisions, the depths of which are substantially equal to the depth of the motifs in relief, the Applicant has proposed (see publication WO 98/35842), to effect discrete bridging of said incisions. In the patent application published under the reference EP 858875, there is described a mould for producing a tread provided with such incisions with bridging, this mould comprising two parts, a first part molding the outer face of the tread intended to come into contact with the ground during travel and a second mould part molding the inner face. In the molding configuration, it is stated that a molding element of the first mould part is functionally arranged with a molding element of the second mould part to form one and the same molding element and to define at least one orifice which allows the rubber mix to pass through during molding in order to mould at least one connecting bridge. This mould, although it effectively makes it possible to produce tread patterns comprising incisions and grooves provided with a connecting bridge which provide very good noise results, is costly and difficult to use. It is, in fact, imperative to put the molding elements in place sufficiently precisely on each of the two mould parts in order to effect cooperation of said molding elements.
Furthermore, it is known, for example in application GB 511,271, to produce a tread having a first series and a second series of incisions, the first series being formed of incisions which open on to the running surface of the tread when new and having uniform depths approximately half the height of the motifs in relief of said tread, the second series being formed of incisions which do not open on to the running surface of the tread when new. In order to maintain a good grip performance after partial wear of the tread (affecting the latter over only part of its thickness), the incisions of the second series appear on the new running surface to impart to the tread a grip performance on wet ground which is at least equal to the initial performance. These incisions of the second series which appear on the running surface after partial wear themselves extend over only part of the thickness of the tread or even over the remaining thickness. These incisions are offset longitudinally relative to the incisions of the first series, and may appear on the running surface even before the formers have completely disappeared.
Although it has been noticed that a tread provided with a tread pattern such as described in this latter document had better noise performance compared with the same tread provided with incisions extending over the entire thickness of the motifs in relief, it nevertheless remains that the level of performance obtained is far less than with one and the sane tread pattern provided with a plurality of connecting bridges connecting the main walls of the incisions.
There is a need to obtain a tread pattern which offers satisfactory grip performance whatever the level of wear of this tread and which minimizes the noise emissions during travel of a tire fitted with such a tread when new and also during wear of said tire, while offering the possibility of production by molding which is easy to implement and economic.